The reddish carotenoid pigment astaxanthin is commonly found in nature and is conspicuously displayed by a number of animals. Animals unable to synthesize this pigment rely on dietary intake of this pigment or a pigment precursor.
The red skin and flesh color of naturally occurring salmon and trout is due primarily to astaxanthin, which is usually present as an unbound pigment in these fishes. In nature, marine zooplankton and nekton in the diet provide salmon with their carotenoid pigments.
Due to a lack of dietary astaxanthin, fish raised on fish-farms or in hatcheries are generally pale and lack the skin and flesh colors characteristic of fish grown in their natural environment. Whether or not the carotenoids are nutritionally important in the animal or human diet has not been determined, but pigments do make certain foods attractive. That is, since the color of a food is frequently an indicator of its quality, there is a strong consumer preference for fish having natural coloration, even though nutritionally the pale farm produced fish may be identical to those grown in their natural environment. There is also evidence that astaxanthin or its precursor contributes to the distinctive flavor of baked salmon.
Recent increasing concern over health risks has resulted in a ban on various synthetic coloring agents which have a potential of carcinogenicity and/or teratogenicity. Yellow and red azo dyes which are increasingly prohibited from use in foods are being replaced with non-toxic carotenoids. The carotenoids are generally not toxic even at high levels. Thus, naturally occurring carotenoids are the preferred pigment for coloring, e.g. salmonids.
In the past, numerous studies have been carried out utilizing carotenoid containing crustaceans or crustacean processing wastes in salmonid diets. The pale color of fish produced on fish-farms or in hatcheries is improved when the fish are fed a diet supplemented by large quantities of dried, ground-up exoskeletal crustacean remains. Crustacean shells are, however, very low in carotenoid content and high in minerals which, without extensive processing to improve their dietary quality, restricts their inclusion in salmonid diets. Further, a satisfactory color can be developed in this manner of feeding only over long periods of time and it is desirable, if not indeed essential in the economic sense, to develop satisfactory colors within very short periods of time.
It is known that astaxanthin per se can be added to fish food to improve fish color. For example, U.S. Pat. No. 4,239,782 describes a method for enhancing the color of fish comprising adding to the fish food a pigmenting agent such as astaxanthin and additionally limited amounts of the hormone testosterone, which hormone acts as a catalyst in combination with a chosen pigmenting agent or agents to enhance the color of fish.
The two primary commercial sources for astaxanthin per se are extracts from crustacean shells and chemical synthetics. The red carotenoid pigment can be extracted from the exoskeletal crustacean shells and tissues and fed, admixed with other feed in dietary formulations, to the farm fish, crustacea and certain fowls in massive concentrations to develop satisfactory skin, flesh, carapace or egg yolk pigmentations. Examples of processes for extraction of astaxanthin from crustacean shell and tissue waste are described, for example, in U.S. Pat. Nos. 3,906,112 (Anderson) and 4,505,936 (Meyers et al).
In an article in Journal of Food Science, Volume 47 (1982), entitled "Extraction of Astaxanthin Pigment from Crawfish Waste Using a Soy Oil Process", various extraction techniques are described. For example, whole crawfish waste is ground-up, the comminuted crawfish waste is admixed with water, the pH is adjusted with an alkali or acid, an enzyme is added to the solution, and the solution stirred, heated and hydrolyzed. After hydrolysis, the astaxanthin is extracted with oil and the astaxanthin enriched oil recovered by centrifugation. However, the cost of natural isolates of astaxanthin, especially from krill and crawfish shells, can cost anywhere from $5,000 to $15,000 per kilogram. Obviously, a less source dependent and more economical process for production of astaxanthin is needed.
Pigmentation of salmon and trout flesh has also been accomplished using the synthetic carotenoid canthaxanthin as a feed additive, but this chemical is rather expensive and has been reported to produce a somewhat unsatisfactory color in salmonids. Recent work in chemical synthesis of astaxanthin is exemplified by U.S. Pat. No. 4,245,109 (Mayer et al). 4,283,559 (Broger et al), and 4,585,885 (Bernhard et al). The present cost of synthetic astaxanthin pigment is approximately $2,000 per kilogram. Many countries, however, prohibit the use of synthetic carotenoids.
Astaxanthin remains one of the most expensive ingredients used in salmon feed for pen-reared salmon. As the interest in aquaculture, i.e., farming fish, has exploded recently, the commercial demand for an economical source of astaxanthin has grown proportionately.
The pigmentation of avian egg yolks has also been studied because of the economic importance of color in chicken egg yolks. Yolks with a high pigment content are demanded. The most common pigment source in commercial diets has been yellow corn, which supplies the prominent egg yolk pigments cryptoxanthin, zeaxanthin and lutein. Unfortunately, higher energy grains such as milo, wheat, rice and barley are replacing corn in the chicken diet, with the consequent loss in pigmentation. Astaxanthin can be used as a poultry food supplement to increase yolk pigmentation.
One approach not presently commercially employed in production of astaxanthin is biosynthesis, i.e., employment of microorganisms to synthesize astaxanthin.
As a microbial source of astaxanthin, the yeast Phaffia rhodozyma is known (Johnson, "Astaxanthin Production by the Yeast Phaffia rhodozyma and its use as a Pigment Source in Animal Feeding". Masters Thesis, University of California at Davis, 1976). Yeast is generally considered to be a highly nutritious feedstuff and is often desirable in animal diets; the additional attribute of containing astaxanthin suggests that P. rhodozyma may be an ideal feed supplement for animals that require a dietary source of this pigment, e.g. salmonids, crustaceans, laying hens, or birds such as flamingoes.
However, the pigment yield of naturally occurring P. rhodozyma is only in the order of 200 to 600 ppm/dry weight of yeast in 6 day growth. As a source of astaxanthin per se, naturally occurring P. rhodozyma is inadequate. Experiments in supplementing the diets of certain fish, crustaceans and fowl with naturally occurring P. rhodozyma showed some promise. Practically, however, the large volume of P. rhodozyma which must be added as a food supplement in order to obtain satisfactory levels of pigmentation detracts from the commercial suitability of naturally occurring P. rhodozyma as a pigment source.